The use of carbon dioxide for enhanced production of oil and gas from reservoirs is well known. Usage of liquid carbon dioxide (LCO2) in fracture treatment of oil and gas formations has certain advantages in water sensitive and low pressure formations including a significant reduction of water volume and promotion of water flowback, (retrieval of water/fluid used in fracture treatment) which minimizes formation damage caused by water. LCO2 used in fracturing treatments is typically added to a high pressure stream of water and proppant (typically sand) at the well-head. Combining water with proppant and adding a separate pressurized LCO2 stream is the most conventional method of forming a CO2-energized fracture fluid. This is due, in large part, because it is simpler to mix proppant with water at atmospheric pressure, than it is to add proppant to liquid carbon dioxide at a pressure above the triple point of carbon dioxide, (i.e., greater than 75.1 psia).
Equipment is available and can be used for small fracture treatments to deliver about 100 percent LCO2 and essentially dry proppant. In this case, “small” fracture treatments are considered to be those up to about 20 tons of proppant on a per batch basis. This equipment is designed for the delivery of a dry proppant and LCO2 combination and typically delivers the mixture from a relatively small pressurized batch tank. Additional quantities of LCO2 are added to dilute the proppant concentration of the fracturing slurry stored in the tank to the appropriate level required for fracture treatment. Once the small batch of LCO2 and proppant is exhausted, the fracture treatment must either be completed or stopped because it is not possible to quickly empty and refill these existing relatively “small” batch vessels.
Therefore, a need has been established to develop a portable LCO2 and proppant blender system that possesses higher proppant capacity for much larger fracture treatments. One example includes the need for supplying this type of proppant for fracturing long-reach, multi-stage, horizontally drilled formations. Because millions of pounds of proppant may be required for large fracture treatments, a single blender, or multiple blenders must be capable of delivering proppant and LCO2 in a continuous or semi-continuous manner. There also exists a continuing need for and efficient, portable LCO2 and proppant blender that can be used for small fracture treatments.
Earlier efforts, for example, as described in U.S. Pat. No. 4,374,545, provides for a simple batch process where proppant is cooled to LCO2 temperatures by direct contact with LCO2 yielding a batch tank of proppant and LCO2 fracturing slurry. As the proppant slurry is delivered from this batch tank, LCO2 is added to maintain a relatively high LCO2 level in the tank. Using this slurry batch approach to achieve a much larger continuous or semi-continuous injection method is problematic, both because of the long delay and wasted carbon dioxide when the slurry vessel must be “blown down” (i.e., depressurized) to add more proppant. Other systems utilizing LPG (liquefied petroleum gas—such as described in U.S. Pat. Nos. 4,407,825 and 7,735,551) blend dry proppant with flowing LPG at close to ambient temperature. LPG is higher boiling and it is not necessary to precool the proppant and there is no possibility of vaporizing the LPG fluid as dry proppant is added to it.
In order to achieve the established need for a semi-continuous or continuous fracturing operation using a mixture of LCO2 and dry proppant, the system requires subcooling LCO2 (using booster pumping and/or using a subcooling heat exchanger(s) and/or pressurizing the headspace in the supply LCO2 tanks). The subcooling must be managed in such a way so that warm dry proppant can be added to the flowing LCO2 stream without vaporizing a portion of the LCO2 and so that the slurry fracture fluid is available with the appropriate level of net positive suction head (NPSH) required for safe and reliable operation of the high pressure frac pumps. A method for metering dry proppant is also required which is provided by the use of an auger, control valve, eductor, or some other appropriate method of metering proppant. The present invention described below addresses the design of the system, and associated equipment that meets this need.